Background: Low intensity pulsed ultrasound (LIPU) in conjunction with intravenous microbubbles can transiently and reversibly disrupt the blood-brain barrier (BBB), allowing for an increase in the tissue concentration of chemotherapy agents in the brain. Mass spectrometry data from preclinical models (mouse, swine) showed a > 5x enhancement in carboplatin brain concentrations, which correlated well with the spatial distribution of a Gadolinium (Gd) contrast agent used for magnetic resonance imaging (MRI). Methods: The primary objective of this phase I/IIa study (NCT03744026) was to demonstrate the safety of BBB disruption using LIPU in patients with recurrent glioblastoma. This study was a 3+3 design using escalating numbers (3, 6, 9) of activated 1 MHz ultrasound emitters. Nine patients were treated in the escalation phase and another 12 patients were treated with 9 emitters in the expansion phase. Eligibility included recurrent GBM (any recurrence) with a maximum tumor size of < 70 mm. The SonoCloud-9 device (CarThera, Paris, France) was implanted during tumor debulking/resection surgery and replaced the bone flap, with the device targeting the tumor and surrounding peritumoral brain. The device was activated every four weeks for a duration of 270 seconds, concomitantly with IV DEFINITY microbubbles (10 ml/kg), to disrupt the BBB prior to administration of carboplatin (AUC 4-6). MRI was performed to verify safety and evaluate efficacy of BBB disruption with Gd enhancement. Results: No DLTs were observed. The overall tolerance of the SonoCloud-9 implant was good, with two transient, manageable grade 3 wound infections and one grade 1 acquired meningocele event considered as probably related to the overall procedure. The most frequent neurologic adverse events were grade 1 blurred vision (5%) and dizziness (5%). Conclusions: Significant Gd enhancement was observed after more than 90% of sonication sessions, suggesting effective BBB disruption and carboplatin enhancement. Clinical trial information: NCT03744026